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Understanding Mendel's Laws of Genetics

Explore the science of inheritance, from ancient breeding practices to Mendel's experiments with pea plants, revealing the laws of segregation and independent assortment. Discover how genes determine heritable traits and the principles of genetic inheritance.

michelejones
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Understanding Mendel's Laws of Genetics

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  1. Chapter 9 0 Patterns of Inheritance

  2. Purebreds and Mutts–A Difference of Heredity • Purebred dogs • Are very similar on a genetic level due to selective breeding

  3. Mutts, or mixed breed dogs on the other hand • Show considerably more genetic variation

  4. MENDEL’S LAWS • 9.1 The science of genetics has ancient roots • The historical roots of genetics, the science of heredity • Date back to ancient attempts at selective breeding

  5. Petal Stamen Carpel Figure 9.2 A Figure 9.2 B • 9.2 Experimental genetics began in an abbey garden • Modern genetics • Began with Gregor Mendel’s quantitative experiments with pea plants

  6. Mendel crossed pea plants that differed in certain characteristics • And traced traits from generation to generation 1Removed stamens from purple flower White Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower Parents(P) Purple 3 Pollinated carpel matured into pod 4 Planted seeds from pod Offspring(F1) Figure 9.2 C

  7. Purple Flower color White Terminal Axial Flower position Green Yellow Seed color Seed shape Round Wrinkled Pod shape Inflated Constricted Green Yellow Pod color Tall Stem length Dwarf • Mendel hypothesized that there are alternative forms of genes • The units that determine heritable traits Figure 9.2 D

  8. P generation (true-breedingparents)  Purple flowers White flowers F1 generation All plants havepurple flowers Fertilizationamong F1 plants(F1 F1) F2 generation 3 4 1 4 of plantshave white flowers of plantshave purple flowers • 9.3 Mendel’s law of segregation describes the inheritance of a single characteristic • From his experimental data • Mendel deduced that an organism has two genes (alleles) for each inherited characteristic Figure 9.3 A

  9. For each characteristic • An organism inherits two alleles, one from each parent

  10. If the two alleles of an inherited pair differ • Then one determines the organism’s appearance and is called the dominant allele • The other allele • Has no noticeable effect on the organism’s appearance and is called the recessive allele

  11. P plants Genetic makeup (alleles) pp PP Gametes All p All P F1 plants (hybrids) All Pp 1 2 1 2 P p Gametes Sperm p P F2 plants Phenotypic ratio 3 purple : 1 white Pp P PP Eggs Genotypic ratio 1 PP: 2 Pp: 1 pp Pp p pp • Mendel’s law of segregation • Predicts that allele pairs separate from each other during the production of gametes Figure 9.3 B

  12. Dominantallele Gene loci a B P a b P Recessiveallele Genotype: PP aa Bb Heterozygous Homozygousfor thedominant allele Homozygousfor therecessive allele • 9.4 Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene • Reside at the same locus on homologous chromosomes Figure 9.4

  13. 9.5 The law of independent assortment is revealed by tracking two characteristics at once • By looking at two characteristics at once • Mendel tried to determine how two characteristics were inherited

  14. Hypothesis: Independent assortment Hypothesis: Dependent assortment RRYY P generation rryy RRYY rryy ry ry Gametes Gametes RY  RY RrYy RrYy F1 generation Sperm Sperm 1 4 1 4 1 4 1 4 ry ry RY RY 1 2 1 2 ry RY 1 4 RY 1 2 RY RrYY RRYY RRYy RrYy F2 generation Eggs 1 4 ry 1 2 ry rrYY rrYy RrYy RrYY Eggs Yellowround 9 16 1 4 Ry RrYy RRyy RRYy Rryy Greenround 3 16 1 4 ry Yellowwrinkled Actual resultscontradict hypothesis 3 16 rryy RrYy rrYy Rryy Greenwrinkled 1 16 Actual resultssupport hypothesis • Mendel’s law of independent assortment • States that alleles of a pair segregate independently of other allele pairs during gamete formation Figure 9.5 A

  15. Blind Blind Phenotypes Genotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Mating of heterozygotes (black, normal vision) BbNn  BbNn 9 black coat, normal vision 3 black coat, blind (PRA) 1 chocolate coat, blind (PRA) 3 chocolate coat, normal vision Phenotypic ratio of offspring Figure 9.5 B • An example of independent assortment

  16. Testcross: Genotypes bb B_ Two possibilities for the black dog: BB or Bb Gametes B b B b Bb b bb Bb 1 black : 1 chocolate All black Offspring • 9.6 Geneticists use the testcross to determine unknown genotypes • The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual • Can reveal the unknown’s genotype Figure 9.6

  17. 9.7 Mendel’s laws reflect the rules of probability • Inheritance follows the rules of probability

  18. F1 genotypes Bbmale Formation of sperm Bbfemale Formation of eggs 1 2 1 2 b B b B B B 1 2 B 1 4 1 4 F2 genotypes 1 2 B b b b b 1 4 1 4 • The rule of multiplication • Calculates the probability of two independent events • The rule of addition • Calculates the probability of an event that can occur in alternate ways Figure 9.7

  19. Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe CONNECTION • 9.8 Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits • Follows Mendel’s laws Figure 9.8 A

  20. D ? John Eddy Dd Abigail Linnell D ? Hepzibah Daggett Dd Joshua Lambert dd Jonathan Lambert Dd Elizabeth Eddy D ? Abigail Lambert Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing • Family pedigrees • Can be used to determine individual genotypes Figure 9.8 B

  21. Table 9.9 CONNECTION • 9.9 Many inherited disorders in humans are controlled by a single gene • Some autosomal disorders in humans

  22. Parents Normal Dd Normal Dd  Sperm Dd Dd Normal (carrier) DD Normal D Offspring Eggs Dd Normal (carrier) dd Deaf d • Recessive Disorders • Most human genetic disorders are recessive Figure 9.9 A

  23. Dominant Disorders • Some human genetic disorders are dominant Figure 9.9 B

  24. CONNECTION • 9.10 New technologies can provide insight into one’s genetic legacy • New technologies • Can provide insight for reproductive decisions

  25. Identifying Carriers • For an increasing number of genetic disorders • Tests are available that can distinguish carriers of genetic disorders

  26. Chorionic villus sampling (CVS) Amniocentesis Needle inserted through abdomen to extract amniotic fluid Ultrasound monitor Ultrasound monitor Suction tube inserted through cervix to extract tissue from chorionic villi Fetus Fetus Placenta Placenta Chorionic villi Uterus Cervix Cervix Uterus Amniotic fluid Centrifugation Fetal cells Fetal cells Biochemical tests Several weeks Several hours Karyotyping Figure 9.10 A • Fetal Testing • Amniocentesis and chorionic villus sampling (CVS) • Allow doctors to remove fetal cells that can be tested for genetic abnormalities

  27. Fetal Imaging • Ultrasound imaging • Uses sound waves to produce a picture of the fetus Figure 9.10 B

  28. Newborn Screening • Some genetic disorders can be detected at birth • By simple tests that are now routinely performed in most hospitals in the United States

  29. Ethical Considerations • New technologies such as fetal imaging and testing • Raise new ethical questions

  30. VARIATIONS ON MENDEL’S LAWS • 9.11 The relationship of genotype to phenotype is rarely simple • Mendel’s principles are valid for all sexually reproducing species • But genotype often does not dictate phenotype in the simple way his laws describe

  31. P generation Red RR White rr  r R Gametes F1 generation Pink Rr Genotypes: 1 2 1 2 HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors r R Gametes Sperm Phenotypes: 1 2 1 2 r R LDL Pink rR LDL receptor 1 2 Red RR R Eggs F2 generation Pink Rr White rr 1 2 Cell r Mild disease Severe disease Normal • 9.12 Incomplete dominance results in intermediate phenotypes • When an offspring’s phenotype is in between the phenotypes of its parents • It exhibits incomplete dominance Figure 9.12 A Figure 9.12 B

  32. 9.13 Many genes have more than two alleles in the population • In a population • Multiple alleles often exist for a characteristic

  33. Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Blood Group (Phenotype) Antibodies Present in Blood Genotypes O A B AB Anti-A Anti-B ii O IAIA or IAi Anti-B A IBIB or IBi Anti-A B — AB IAIB • The ABO blood type in humans • Involves three alleles of a single gene • The alleles for A and B blood types are codominant • And both are expressed in the phenotype Figure 9.13

  34. Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells 5,555 Clumping of cells and clogging of small blood vessels Breakdown of red blood cells Accumulation of sickled cells in spleen Brain damage Pain and fever Spleen damage Damage to other organs Physical weakness Heart failure Anemia Impaired mental function Pneumonia and other infections Kidney failure Paralysis Rheumatism • 9.14 A single gene may affect many phenotypic characteristics • In pleiotropy • A single gene may affect phenotype in many ways Figure 9.14

  35. P generation aabbcc (very light) AABBCC (very dark)  F1 generation AaBbCc AaBbCc 20 64 15 64 15 64 1 64 6 64 1 64 6 64 Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 20 64 1 8 F2 generation 1 8 15 64 1 8 1 8 Fraction of population Eggs 1 8 6 64 1 8 1 8 1 64 1 8 Skin color • 9.15 A single characteristic may be influenced by many genes • Polygenic inheritance • Creates a continuum of phenotypes Figure 9.15

  36. 9.16 The environmental affects many characteristics • Many traits are affected, in varying degrees • By both genetic and environmental factors Figure 9.16

  37. CONNECTION • 9.17 Genetic testing can detect disease-causing alleles • Predictive genetic testing • May inform people of their risk for developing genetic diseases

  38. THE CHROMOSOMAL BASIS OF INHERITANCE • 9.18 Chromosome behavior accounts for Mendel’s laws • Genes are located on chromosomes • Whose behavior during meiosis and fertilization accounts for inheritance patterns

  39. All round yellow seeds(RrYy) F1 generation R 14 14 14 14 RY Ry rY ry y r Y r R r R Metaphase Iof meiosis(alternative arrangements) y Y y Y r R r R Anaphase Iof meiosis Y y y Y r r R R Metaphase IIof meiosis y y Y Y y Y Y Y y y Y y Gametes R r r R r R r R Fertilization among the F1 plants : 3 F2 generation 9 : 3 : 1 (See Figure 9.5A) • The chromosomal basis of Mendel’s laws Figure 9.18

  40. Experiment Purple flower PpLI PpLI Long pollen • Observed Prediction • Phenotypes offspring (9:3:3:1) Purple long Purple round Red long Red round 215 71 71 24 284 21 21 55 Explanation: linked genes P L Parental diploid cell PpLI P I Meiosis Most gametes P L P I Fertilization Sperm P I P L P L P L P L Most offspring P L P I Eggs P I P I P I P I P L 3 purple long : 1 red round Not accounted for: purple round and red long • 9.19 Genes on the same chromosome tend to be inherited together • Certain genes are linked • They tend to be inheritedtogether because they reside close together onthe same chromosome Figure 9.19

  41. A B a b A B a b A b a B Crossing over Tetrad Gametes • 9.20 Crossing over produces new combinations of alleles • Crossing over can separate linked alleles • Producing gametes with recombinant chromosomes Figure 9.20 A

  42. Thomas Hunt Morgan • Performed some of the early studies of crossing over using the fruit fly Drosophila melanogaster Figure 9.20 B

  43. Experiment Black body, vestigial wings Gray body, long wings (wild type)  GgLI ggll Male Female Offspring Gray long Black long Black vestigial Gray vestigial 965 944 206 185 Parental phenotypes Recombinant phenotypes 391 recombinants Recombination frequency = = 0.17 or 17% 2,300 total offspring Explanation g l G L ggll (male) GgLI (female) g g l l g g g G L l G l L l Eggs Sperm g g L G L G l l g g g g l l l l Offspring • Morgan’s experiments • Demonstrated the roleof crossing over in inheritance Figure 9.20 C

  44. 9.21 Geneticists use crossover data to map genes • Morgan and his students • Used crossover data to map genes in Drosophila Figure 9.21 A

  45. Mutant phenotypes Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Short aristae Chromosome g l c 17% 9% 9.5% Recombination frequencies Normal wings (L) Red eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Wild-type phenotypes • Recombination frequencies • Can be used to map the relative positions of genes on chromosomes. Figure 9.21 B Figure 9.21 C

  46. (male) (female) 44 + XX 44 + XY Parents’ diploid cells 22 + X 22 + X 22 + Y Egg Sperm 44 + XX Offspring (diploid) 44 + XY SEX CHROMOSOMES AND SEX-LINKED GENES • 9.22 Chromosomes determine sex in many species • In mammals, a male has one X chromosome and one Y chromosome • And a female has two X chromosomes Figure 9.22 A

  47. The Y chromosome • Has genes for the development of testes • The absence of a Y chromosome • Allows ovaries to develop

  48. 22 + X 22 + XX 76 + ZW 76 + ZZ 32 16 • Other systems of sex determination exist in other animals and plants Figure 9.22 B Figure 9.22 C Figure 9.22 D

  49. 9.23 Sex-linked genes exhibit a unique pattern of inheritance • All genes on the sex chromosomes • Are said to be sex-linked • In many organisms • The X chromosome carries many genes unrelated to sex

  50. In Drosophila • White eye color is a sex-linked trait Figure 9.23 A

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